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In an environment that had a pre-existing Exchange Server 2010 SP3 (multiple distributed servers running Mailbox (M), Hub Transport (HT), Client Access (CAS), and Unified Messaging (UM) roles) and new configured hybrid integration with Exchange Online (ExO) in Office 365 tenant, migrated mailbox users were not able to access / view any of the legacy public folders homed on-premises in Exchange 2010.

Resolution

Here are the troubleshooting steps that we have done to make the legacy Exchange 2010 public folders visible to Office 365 ExO migrated mailbox users.

Step 1: Make remote public folders discoverable

If your public folders are on Exchange 2010 or later servers, you must install the Client Access server (CAS) role on all mailbox servers that have a public folder database. This allows the Microsoft Exchange RpcClientAccess service to be running so that all clients can access public folders. For more information, see Install Exchange Server 2010.

Create an empty mailbox database on each public folder server. For Exchange 2010, run the following command. This command excludes the mailbox database from the mailbox provisioning load balancer. This prevents new mailboxes from being added automatically to this database.

Note: We recommend that the only mailbox that you add to this database is the proxy mailbox that you'll create in step 3. No other mailboxes should be created on this mailbox database.

Create a proxy mailbox within the new mailbox database and hide the mailbox from the address book. The SMTP of this mailbox will be returned by AutoDiscover as the DefaultPublicFolderMailbox SMTP, so that by resolving this SMTP the client can reach the legacy exchange server for public folder access.